The UN Secretary-General has withdrawn his invitation to Iran to join this week's peace conference on Syria.

Ban Ki-moon was forced to act after the exiled Syrian opposition threatened to withdraw from the talks if Iran - a major backer of Syrian President Bashar al Assad - takes part in the conference.

It came as a group of top lawyers published what they described as "clear evidence" of the "systematic torture and killing" of up to 11,000 detainees by the agents of the Syrian government.

The lawyers, who are all former international war crimes prosecutors, were handed 55,000 images smuggled out of the country by an ex-military policeman.

In a report, they said the photographs showed emaciated corpses with severe injuries and would support findings of crimes against humanity by the Assad regime.

Some of the victims appeared to have been beaten, strangled or electrocuted, they claimed.

Image Caption:More than 100,000 people have been killed in the Syrian war

Mr Ban said the Iranian foreign minister Mohammad Javad Zarif had repeatedly assured him that he "understood and supported" the aim of the peace conference.

But Iran's UN ambassador declared his country would not join the peace talks, due to open in Switzerland on Wednesday, if it was required to accept a Syria declaration adopted by major world powers in Geneva in 2012.

The roadmap calls for a transitional government to guide the country out of the three-year war which the UN says has killed more than 100,000 people.

During a news conference in New York, UN spokesman Martin Nesirky said: "The Secretary-General is deeply disappointed by Iranian public statements today that are not at all consistent with that stated commitment."

The Syrian National Coalition has said it will join the talks now that Iran's invitation has been retracted.

Spokesman Monzer Akbik said: "We appreciate the United Nations and Ban Ki-moon's understanding of our position. We think they have taken the right decision."

High-ranking delegations from around 40 countries - including Russia, which said the decision to pull Iran's invitation was a mistake but not a catastrophe - are attending the talks.

However, Abbas Araqchi, Iran's deputy foreign minister, said a "comprehensive solution" would not be found unless "all influential parties" are involved in negotiations.

"Everyone knows that without Iran the chances of a real solution to Syria are not that great," he said.

Face-to-face negotiations between the Syrian government and its opponents - the first since the conflict began three years ago - are to start on Friday in Geneva.